Comment by susam

Yes, '?:' is also known as the Elvis operator [1][2]. I sometimes use it in other languages such as Groovy. But I don't use it in C because this happens to be a GCC extension [3][4] and I've often had to compile my C projects with compilers that do not support GCC extensions. The C standard [5] defines the conditional operator as:

  conditional-expression:
    logical-OR-expression
    logical-OR-expression ? expression : conditional-expression

So per the C standard there must be an expression between '?' and ':' and an expression cannot be empty text. To confirm this we need to check the grammar for expression, which unfortunately is a little tedious to verify manually due to its deeply nested nature. Here it is:

  expression:
    assignment-expression
    expression , assignment-expression

  assignment-expression:
    conditional-expression
    unary-expression assignment-operator assignment-expression

  unary-expression:
    postfix-expression
    ++ unary-expression
    -- unary-expression
    unary-operator cast-expression
    sizeof unary-expression
    sizeof ( type-name )
    alignof ( type-name )

  assignment-operator: one of
    = *= /= %= += -= <<= >>= &= ^= |=

  ... and so on ...

The recursion goes further many more levels deep but the gist is that no matter whichever branch the parser takes, it expects the expression to have at least one symbol per the grammar. Perhaps an easier way to confirm this is to just have the compiler warn us about it. For example:

  $ cat foo.c
  int main(void) {
      if () {}
  }

  $ clang -std=c17 -pedantic -Wall -Wextra foo.c && ./a.out
  foo.c:2:9: error: expected expression
      2 |     if () {}
        |         ^
  1 error generated.

Or more explicitly:

  $ cat bar.c
  #include <stdio.h>
  int main(void) {
      printf("%d\n", 0 ?: 99);
      printf("%d\n", 1 ?: 99);
  }

  $ clang -std=c17 bar.c && ./a.out
  99
  1

  $ clang -std=c17 -pedantic bar.c && ./a.out
  bar.c:3:23: warning: use of GNU ?: conditional expression extension, omitting middle operand [-Wgnu-conditional-omitted-operand]
      3 |     printf("%d\n", 0 ?: 99);
        |                       ^
  bar.c:4:23: warning: use of GNU ?: conditional expression extension, omitting middle operand [-Wgnu-conditional-omitted-operand]
      4 |     printf("%d\n", 1 ?: 99);
        |                       ^
  2 warnings generated.
  99
  1

[1] https://kotlinlang.org/docs/null-safety.html#elvis-operator

[2] https://groovy-lang.org/operators.html#_elvis_operator

[3] https://gcc.gnu.org/onlinedocs/gcc/Syntax-Extensions.html

[4] https://gcc.gnu.org/onlinedocs/gcc/Conditionals.html

[5] https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3299.pdf